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+--- Thread: Airbag jackets? Your thoughts? (now other gear questions) (/showthread.php?tid=3085)
- .RJ - 10-15-2007
Evan Wrote:newtons first law would only apply if the rider separated from the motorcycle before the accident.
Not exactly - if the rider ejects from the bike, he's going to maintain speed unless a force is exerted to slow (or accelerate) them. In this case air resistance, gravity, and road rash (titty rash?) will slow him down... but launching off the bike wont speed him up.
- HAULN-SS - 10-15-2007
No one is arguing that the rider doesnt have a projectile like motion after he leaves the bike, dummy. I am saying that as the bike rotates over the front wheel (back wheel coming up and over), like a one armed catapult. Lots of tangential speed could be built up there.
- .RJ - 10-15-2007
HAULN-SS Wrote:Lots of tangential speed could be built up there.
You're kidding, right?
You cant accelerate something without some additional input of force.
- HAULN-SS - 10-15-2007
the handlebars in that kind of crash are going slower than the taillight..what's wrong with you?
- .RJ - 10-15-2007
And whats your point? The rider is ejected from the bike - if the bike is going 100mph (for example) and is instantaneously stopped to 0mph (also, for example), with the rider flying off it he's still going at 100mph no matter what the bike is doing.
- Ginger - 10-15-2007
I don't see how Derek's logic allows the rider to travel faster than the initial velocity.
- .RJ - 10-15-2007
Derek's logic assumes that bike acts as a catapult to 'launch' the rider as it pivots on the front wheel, but that would mean the bike is now going faster than the rider (to apply an accelerative force) being launched off of it, which is impossible.
Deersty - for someone who is a math major you have a pretty poor grasp of basic physics.
- Ginger - 10-15-2007
.RJ Wrote:Derek's logic assumes that bike acts as a catapult to 'launch' the rider as it pivots on the front wheel, but that would mean the bike is now going faster than the rider (to apply an accelerative force) being launched off of it, which is impossible.
Yeah, I was like "where is that force coming from?" but now it seems like it's coming from nowhere
- .RJ - 10-15-2007
asteele2 Wrote:Yeah, I was like "where is that force coming from?"
The vacuum of Deersty's head?
- HAULN-SS - 10-15-2007
I don't think either of you guys understand at all.
<!-- m --><a class="postlink" href="http://www.phy.cmich.edu/people/andy/Physics110/Book/Chapters/Chapter6.htm">http://www.phy.cmich.edu/people/andy/Ph ... apter6.htm</a><!-- m -->
read this, especially the part where V=omega(radius)
- BLINGMW - 10-15-2007
rider trebuchet!
- .RJ - 10-15-2007
HAULN-SS Wrote:I don't think either of you guys understand at all.
You sure about that?
How is the bike going to accelerate the rider as he's launched off of it when both are going the same speed? The only difference is the bike stops, and the rider doesnt.... what is so hard for you to understand?
- mrbaggio - 10-15-2007
HAULN-SS Wrote:No one is arguing that the rider doesnt have a projectile like motion after he leaves the bike, dummy. I am saying that as the bike rotates over the front wheel (back wheel coming up and over), like a one armed catapult. Lots of tangential speed could be built up there.
You are missing an additional force to rotate the bike/rider. The only force involved, if I understand your idea, is the forward moton of the bike. The rear end of the bike will not rotate faster than the bike was traveling if the front end was to come to a quick stop.
- mrbaggio - 10-15-2007
deleted post?
- Evan - 10-16-2007
.RJ Wrote:Evan Wrote:newtons first law would only apply if the rider separated from the motorcycle before the accident.
Not exactly - if the rider ejects from the bike, he's going to maintain speed unless a force is exerted to slow (or accelerate) them. In this case air resistance, gravity, and road rash (titty rash?) will slow him down... but launching off the bike wont speed him up.
I agree with your point, but newtons first law still doesnt (directly) apply. The first law applies to objects with no net forces. The accident itself is most definately a force that is affecting the motion of the rider. You cant say that in an accident the rider will continue on the same path when he gets in a wreck. The 2nd law is more applicable where you take into account the force vector of the wreck.
ISAT 142 flashbacks!
Also, I dont agree with Derek's conclusion that the rider could be accelerated, but the logic isnt so far off to deserve insults from you guys.
In concept, centripetal acceleration could be applied using the front wheel as a point of rotation and the bike as a lever arm rotating faster the longer the lever arm is and 'catapulting' the rider.
Just like a high-side.
The law of conservation of energy is probably where the theory of accelerating the rider falls down, but who knows, there are a lot of forces in the equation and they are not only in motion.
We have all seen high-sides where the rider is tossed further than the bike so while I dont think it applies here the insults are most definately not warranted to this line of thinking.
- Maengelito - 10-16-2007
Evan Wrote:ISAT 142 flashbacks!
ugh, i hate you evan
- .RJ - 10-16-2007
Evan Wrote:I agree with your point, but newtons first law still doesnt (directly) apply. The first law applies to objects with no net forces.
The way I looked at it, the bike is moving along at a given speed, so there are no forces acting on it (or the rider) to accelerate it faster or slower. Maybe not quite 'at rest' but not changing.
- PDenbigh - 10-16-2007
I'll throw my hat in this ring. I can see points to all of these arguments, but my conclusion is that the pivoting of the bike has no effect on the horizontal speed of the rider.
This all takes place within a thousandth of a second:
Phase 1: Rider: When he first hits, his arms bend and he is launched forward for a few millionths of a second. A space forms between him and the bike seat because the crunching of the stronger frame bike causes the middle/back of the bike to slow down at a slightly slower rate. He continues moving at the same speed he was traveling, minus any acceleration (ÔÇ£decelerationÔÇØ) that occurred due to breaking bones, wind resistance, etc. He started decelerating the moment the tire contacts the solid object. ThatÔÇÖs why it takes 180hp to go 140mph in the real world ÔÇô itÔÇÖs equalizing (or exceeding) the other forces pushing against him.
Phase 2: Bike and Catapult: The bottom line is everything happens so quickly that no noticeable positive acceleration would take place. The only thing that happens is that the force vector changes from purely horizontal to some vertical, and thus why he is launched upward.
The length of the bike is the radius of a circle. Take something tied to a string and spin it around your finger. If you spin the string but donÔÇÖt let it coil around your finger, in other words you keep a constant radius, the object doesnÔÇÖt speed up. Now, if you let it coil around your finger, thus decreasing the radius constantly until the object hits your finger, the speed of the end object increases. The only way the rear half of the bike could actually speed up is if the distance between the back and the front decreased. Technically, it does because itÔÇÖs getting smashed, but it is such a small amount that it really wouldnÔÇÖt matter. Because itÔÇÖs such a small amount of horizontal acceleration, it wouldnÔÇÖt be enough to close the horizontal gap formed between the rider and bike in Phase 1. The only way he could be horizontally accelerated by the decreased radius catapult is if the bike again made contact to with him, which it doesnÔÇÖt. He is accelerated vertically by the bike because there is no other vertical force acting on him until the bike start pivoting, and thus no vertical space is formed.
- Evan - 10-16-2007
Maengelito Wrote:Evan Wrote:ISAT 142 flashbacks!
ugh, i hate you evan
cute girls in my isat 142 class, so i dont mind a relapse every now and then.
- Evan - 10-16-2007
.RJ Wrote:sure, but the event question is when the bike hits something, no?Evan Wrote:I agree with your point, but newtons first law still doesnt (directly) apply. The first law applies to objects with no net forces.
The way I looked at it, the bike is moving along at a given speed, so there are no forces acting on it (or the rider) to accelerate it faster or slower. Maybe not quite 'at rest' but not changing.